Recently, ionomer resins have been widely used as of cover material for golf balls (e.g. Japanese Patent Kokai Publication No. 49727/1974, etc.). This is because the ionomer resin is superior in rebound characteristics, durability, etc. and it is easily processed. However, since the ionomer resin has high rigidity and hardness, the resulting golf ball is poor in shot feel and controllability in comparison with balata (transpolyisoprene)-covered thread wound golf ball.
In order to improve shot feel and controllability of the ionomer-covered golf balls, it has been attempted to soften the ionomer resin by various means, but golf balls having sufficient performance have not been obtained. For example, Japanese Patent Kokai Publications Nos. 188270/1982, 54928/1994, 79017/1994, and the like propose that the ionomer resin is mixed with a rubber to soften it, but it is difficult to have stable physical properties depending on the variability of the state of crosslinking of the rubber.